It is sometimes necessary to attach a hydraulic breaker as an attachment, instead of a bucket which is usually incorporated as a working unit, to a hydraulic excavator in order to crush a building, a block of rock or the like. In this breaker work, a smaller volume of hydraulic oil, which is approximately one-half of that in the excavating work, is sufficient. Accordingly, the rotational speed of the engine is controlled to N1, N2 or N3 (rev/min) at a predetermined torque T0 by decreasing the volume of fuel injection along engine torque curves shown in FIG. 7, in order to aim at reducing fuel consumption of the engine, thereby controlling the hydraulic oil volume necessary for the breaker work while setting the volume of the hydraulic oil to V (cc/rev), so as to obtain a hydraulic volume of V*N1, V*N2 or V*N3 (cc/min). It is noted that the suction torque T of the hydraulic pump is exhibited by T=kP.times.V where K is a proportional constant, and P is a load pressure of the hydraulic pump, and accordingly, if the volume V (cc/rev) of the hydraulic oil is set to be constant, the load pressure P0 of the hydraulic pump using the torque T0 shown in FIG. 7 as the above-mentioned suction torque is proportional to the above-mentioned suction torque T0. Further, as shown in FIG. 8, a merging change-over circuit for two hydraulic pumps is provided, in which one of two service valves is changed over so as to change the flow rates of first and second pumps in order to control the volumes of hydraulic oil required respectively for an excavation work and a breaker work. That is, referring to FIG. 8, a variable displacement hydraulic pump (which will be simply denoted "main pump") 51, has connected in parallel thereto five control valves for driving actuators for turning, boom Hi, service, arm Lo and leftward running, and a main pump 52 has connected in parallel thereto five selector valves for driving actuators for rightward running, bucket, boom Lo, arm Hi and service. Further, pipe lines 54, 55 connected to outlet ports of a service valve 53 are connected respectively to hydraulic circuits extending from a left side service valve 56 to an actuator 57 for a breaker or the like, and are merged together. A pedal 58 for manipulating an attachment such as the breaker or the like abuts against a pilot pressure control valve 39 (which will be denoted "PPC valve"), using a control pump 40 as a hydraulic pressure source. One of two pilot circuits 41, 42 extending from the PPC valve 39, is connected to a left pilot cylinder belonging to the service valve 56, and the other one of them is connected to a right pilot cylinder of the service valve 56. Further, the pilot circuits 41, 42 are provided respectively with branch circuits 43, 44 which are connected to left and right pilot cylinders belonging to the service valve 53, by way of solenoid type pilot circuit selector valves 45, 46. Solenoids of these pilot circuit selector valves 45, 46 are connected to a change-over switch 50. During normal excavation work or the like, if the actuator 57 requires a flow rate corresponding to two pumps, the change-over switch 50 is manipulated to the turn-on side so that the solenoids of the pilot circuit selector valves 45, 46 are energized, and accordingly, the branch circuits 43, 44 of the pilot circuits 41, 42 are communicated so that the pilot pressure in accordance with a manipulation value to the pedal 58 acts upon the right or left pilot cylinders belonging to the service valve 53 and the service valve 56. Thus, the total flow rates from the main pumps 51, 52 act upon the actuator 57 for driving the attachment. Further, in such a case that a breaker work is carried out by the actuator 57, a flow rate corresponding to one pump is sufficient, and accordingly, the change-over switch 50 is manipulated onto the turn-off side. Accordingly, the solenoids of the pilot circuit selector valves 45, 46 are deenergized so that the branch circuits 43, 44 of the pilot circuits 41, 42 are blocked, and therefore a pilot pressure in accordance with a manipulation degree of the pedal 58 acts upon only the left or right pilot cylinder belonging to the service valve 56. Thus, a flow rate from the main pump 51 alone acts upon the actuator 57 for driving the hydraulic breaker. It is noted that a relief valve 47 and an orifice 48 are provided in the main circuit in order to control the discharge rate of the main pump in accordance with a movement of a spool in each of the control valves 56, and in particular to control the discharge rate of the main pump to a minimum value which can fill up a leakage volume of hydraulic oil so as to reduce the useless flow volume when all control valves are held at their neutral positions, and a flow rate regulating mechanism 49 for the main pump 52 is controlled by a pressure upstream of the orifice 48, thereby the discharge rate of the main pump 52 is controlled. As the control for the discharge rate of the main pump 51 is similar to that for the main pump 52, the explanation thereof is omitted. However, in a method in which the rotational speed of the engine is controlled as shown in FIG. 7 so as to obtain a volume of hydraulic oil required for the breaker work and to aim at reducing the fuel consumption of the engine, the engine rotational speed is lowered so as to set the predetermined load pressure P0 or the absorbing torque T0 of the hydraulic pump to points A1, A2, A3, and accordingly, the discharge rate of the hydraulic pump is decreased to a volume required for the work in order to aim at reducing the fuel consumption of the engine. However, since the above-mentioned points A1, A2, A3 are shifted outwardly from the center of equi-fuel consumption curves as denoted by B (the center gives 100%), the fuel consumption of the engine is deteriorated, and accordingly, it has been difficult to aim at reducing the fuel consumption of the engine. Further, the flow rate changing circuit as shown in FIG. 8 incurs the following problems: (1) Two pilot circuit selector valves 45, 46 have to be provided in the pilot circuit, and further, two pipe lines 54, 55 which are merged to the main circuit extending from the service valve 53 to the actuator 57 for the attachment, are required. Accordingly, the hydraulic circuit is complicated so as to lower the reliability of the hydraulic excavator and to incur an increase in manhours for inspection and maintenance and an increase in the manufacturing cost. (2) Since the flow rate of hydraulic oil fed to the actuator 57 for the attachment is given through the two stage change-over control for one or two pumps, a fine adjustment for the flow rate can not be made.
The present invention is devised in view of the above-mentioned problems inherent to the conventional arrangement, and accordingly, one object of the present invention is to set the volume of hydraulic oil to an optimum value by setting a low load mode so as to subject a hydraulic pump to load sensing control during a breaker work or the like where a smaller volume of hydraulic fluid is sufficient in comparison with a normal excavation work, and to drive an engine for driving the hydraulic pump at a rotational speed at which the fuel consumption is lowered.